Heat exchanger in composite material and method for making same

ABSTRACT

The heat exchanger comprises an intermediate portion ( 14 ) of refractory composite material, e.g. of C/C composite material, in which fluid circulation channels ( 16 ) are formed, and which is interposed between a portion of refractory composite material, e.g. having a ceramic matrix, such as C/SiC composite material, forming a heat shield ( 12 ) and a portion of thermostructural composite material, e.g. C/C composite material, forming a heat exchanger support structure ( 18 ), the component portions of the heat exchanger being assembled together by brazing. The heat exchanger can be used as a wall element exposed to an intense heat flux, in particular in a nuclear fusion reactor or in a ram jet combustion chamber.

FIELD OF THE INVENTION

The invention relates to heat exchangers which use heat exchangeassemblies based on a circulating fluid and which are designed to beemployed in a severe temperature environment.

Particular, but non-limiting, fields of application of the invention aresystems for transforming materials, e.g. nuclear fusion reactors, andpropulsion systems, in particular wall elements for the combustionchambers of jet engines, in particular ram jets.

BACKGROUND OF THE INVENTION

The heat exchangers used in such applications are generally made ofmetal, at least in part. Unfortunately, the thermal and mechanicalproperties of metals and metal alloys limit their field of use, and alsotheir performance and safety. Furthermore, metal heat exchangers areheavy and bulky, which penalizes use thereof, at least in someapplications.

It has been envisaged to use refractory composite materials alone or incombination with metals for making heat exchangers designed to be usedin a severe temperature environment, in particular for the wall of anuclear fusion reactor. Thus, patent application WO 98,/03297 describesmaking such a heat exchanger by brazing pieces of carbon-carbon (C/C)composite material on a metal (copper) substrate cooled by fluidcirculation. That involves using a metal. Also known is U.S. Pat. No.5,583,895 which describes a heat exchanger structure for the sameapplication in the form of a C/C composite material block in which fluidcirculation passages are formed. The walls of the passages are madeleakproof by a metal lining, e.g. made of copper, which is brazed to theC/C composite material.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to provide a heat exchanger capable ofbeing used in a severe temperature environment.

Another object of the invention is to provide a heat exchanger in whichthe various thermal and structural functions can be optimized so thatmass, bulk, and cost are reduced as far as possible.

Another object of the invention is to provide a heat exchanger capableof being made easily.

Another object of the invention is to provide a method of manufacturingsuch a heat exchanger.

A heat exchanger of the invention is characterized in that it comprisesan intermediate portion of refractory composite material in which fluidcirculation channels are formed, the intermediate portion beinginterposed between a portion of refractory composite material forming aheat shield and a portion of thermostructural composite material forminga support structure of the heat exchanger, the portions constituting theheat exchanger being assembled together by brazing.

A thermostructural composite material is a composite material havingmechanical properties making it suitable for constituting structuralelements and which conserves these properties up to high temperatures.Such thermostructural composite materials are typically compositematerials having fiber reinforcement of refractory fibers such as carbonfibers or ceramic fibers, densified by a refractory matrix such as acarbon matrix or a ceramic matrix. Examples of thermostructuralcomposite materials are carbon/carbon (C/C) composite materials withreinforcing fibers and a matrix made of carbon, and ceramic matrixcomposite (CMC) materials, e.g. having a matrix of silicon carbide(SiC).

Advantageously, the thermostructural composite material forming thesupport structure of the heat exchanger is a C/C composite material. Itcan be in the form of a honeycomb or of a composite material in whichthe fiber reinforcement is formed by superposed layers of fibers bondedtogether by fibers extending transversely relative to the layers, as canbe obtained by needling, e.g. as described in patent U.S. Pat. No.4,790,052.

Also advantageously, the material of the intermediate portion is also aC/C composite material which is then used more for its refractoryqualities than for its structural qualities.

It is possible to envisage making the portion that forms the supportstructure and the intermediate portion as a single block of C/Ccomposite material to which the portion forming the heat shield isbrazed.

Also advantageously, the material of the portion forming the heat shieldis a material of the CMC type, e.g a C/SiC or SiC/SiC composite material(i.e. a material having reinforcing fibers of carbon or of siliconcarbide and densified by a silicon carbide matrix), which materials aremore suitable than C/C composite materials for exposure to intense heatflux, particularly in an oxidizing atmosphere. An advantage of the heatexchanger of the invention lies in the possibility of selectingmaterials that are the most suitable for performing the thermalfunctions and for performing the mechanical functions of the heatexchanger, thereby making it possible to optimize manufacture of theheat exchanger in terms of performance and bulk.

According to yet another feature of the heat exchanger of the invention,the fluid circulation channels are formed in one face of theintermediate portion, e.g. by machining, and they are defined in part bythe adjacent wall of one of the other two portions. The fluidcirculation channels are thus particularly simple to make.

If necessary, the channels can be made leakproof by forming a coating ontheir walls, e.g. a thin layer of metal coating. Such a coating can beformed over the entire facing faces of the portions that are to beassembled together so as to facilitate brazing, thereby alsoconstituting an adhesion layer for brazing purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made below to the accompanying drawings, in which:

FIG. 1 is a section through a heat exchanger element constituting afirst embodiment of the invention;

FIG. 2 shows the steps in a method of making the heat exchanger elementof FIG. 1;

FIG. 3 is an exploded view of a jet engine combustion chamber elementforming a heat exchanger that constitutes a second embodiment of theinvention; and

FIG. 4 shows in highly diagrammatic manner a ram jet chamber with adetail view showing a wall element of the combustion chamber forming aheat exchanger constituting a third embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a section view of a unitary block 10 constituting a heatexchanger element. The block 10 can constitute a wall element of anenclosure in which severe thermal conditions exist, e.g. a wall elementof a plasma confinement chamber in a nuclear fusion reactor.

The heat exchanger block 10 comprises a heat shield 12 whose outer face12 a is to be exposed to a heat flux, an intermediate portion 14 havingfluid circulation channels 16, and a support structure 18. Theintermediate portion is interposed between the heat shield 12 and thesupport structure 18 and it is bonded thereto by brazing. The fluidcirculation channels 16 are machined in the face of the intermediateportion that is situated adjacent to the heat shield 12 and that iscovered by the inner face 12 b of the heat shield, which inner face 12 bthus defines the channels 16 in part. The channels 16 are for connectionto a circuit for circulating a cooling fluid.

The heat shield 12 which is exposed to the most severe temperatureconditions is made of a refractory composite material, preferably aceramic matrix composite (CMC) material, e.g. a composite material ofthe C/SiC type, i.e. having carbon fiber reinforcement densified with amatrix of silicon carbide.

The intermediate portion is also made of a refractory compositematerial, e.g. a C/C composite material having carbon fiberreinforcement densified by a carbon matrix.

The support structure is made of a thermostructural composite materialand is designed to provide the structural function of the block 10. Forexample, a support structure can be used in the form of a honeycombstructure of C/C composite material. A method of manufacturing such astructure is described in patent U.S. Pat. No. 5,415,715. It is alsopossible to use a support structure in the form of a C/C compositematerial in which the fiber reinforcement is made up of plane layers offiber fabric bonded together by fibers extending trarsversely relativeto the layers. By way of example, the layers can be layers of wovencloth, unidirectional sheets superposed in different directions, layersof felt, ..., and they are preferably bonded together by needling. Amethod of manufacturing such a C/C composite material is described inpatent U.S. Pat. No. 4,790,052.

FIG. 2 gives the steps in a method of manufacturing the exchanger block10.

The CMC material heat shield, e.g. made of C/SiC composite material, theintermediate portion of C/C composite material, and the supportstructure of C/C composite material are all made separately (steps 20,22, 24). Methods of manufacturing pieces out of composite material ofthe C/C or C/SiC type by preparing a fiber reinforcement or preform, andthen densifying the fiber reinforcement with a matrix are well known.Densification can be performed by chemical vapor infiltration or byimpregnation using a precursor of the matrix in the liquid state andtransforming the precursor by heat treatment.

The channels 16 are machined in one of the faces of the intermediateportion 14 (step 26).

Thereafter, a metal coating can be formed over the facing faces of theintermediate portion, of the heat shield, and of the support structurein their entirety (step 28). The metal coating is selected to improvewettability for the brazing alloy that is used subsequently forassembling the various portions together, and thus to improve adhesionof the brazing alloy. The metal coating also serves to leakproof thewalls of the fluid circulation channels. C/C composite materials or CMCmaterials obtained as mentioned above inevitably present residualporosity and that needs to be closed on the surface in order to ensurethat the channels are leakproof.

The metal coating, e.g. of titanium, chromium, zirconium, hafnium, orberyllium can be deposited by chemical vapor deposition or by vacuumsputtering.

In the event of it being unnecessary to have a metal coating foradhesion of the brazing alloy, it is still necessary to leakproof thewalls of the channels 16. Such leakproofing can then be performed bydepositing a sealing layer at least on the machined portions of theintermediate portion and on the facing portions of the adjacent face ofthe heat shield. The sealing layer can be deposited by chemical vapordeposition. It can be metallic or non-metallic, e.g. it can be carbon orceramic.

Brazing (step 29) is performed by depositing a layer of brazing alloy onthe faces for assembly of the intermediate portion, of the heat shield,and of the support structure, by holding the assembly together intooling, and by raising its temperature to the brazing temperatureappropriate for the brazing alloy used. The alloy used is selected fromthose known for brazing ceramics or refractory compositions to oneanother or to metals, e.g. the alloys sold under the name “TiCuSil” or“Cu-ABA” by the US company Wesgo, Inc. Reference can be made toabove-mentioned patent application WO 98/03297, and to an article by A.G. Foley and D. J. Andrews, “Active metal brazing for joining ceramicsto metals”, GEC Alsthom Technical Review, No. 13, February 1994, France,pp. 49-64.

FIG. 3 is an exploded view of another embodiment of a heat exchanger ofthe invention constituting an element 30 of a jet engine combustionchamber. The heat shield 32 is an axially symmetrical annular piecehaving a cylindrical front portion extended rearwardly by afrustoconical portion. The heat shield 32 is made as a single piece ofCMC composite material, e.g. of C/SiC composite material. The fiberreinforcement of the composite material is made by winding a fiberfabric onto a mandrel of appropriate shape, and the resulting preform isdensified by the matrix of the composite material.

The fluid circulation channels 36 are formed in the axial direction bymachining the face of an intermediate portion 34 that is situated facingthe heat shield 32. The intermediate portion 34 is made of a C/Ccomposite material. The cooling fluid is a fuel which is heated bypassing through the heat exchanger prior to being injected into thecombustion chamber. Fluid admission and outlet orifices 33a and 33b areformed through the shied 32 in the vicinity of its axial ends, and levelwith grooves such as 37 that are machined circumferentially at the frontand at the rear of the intermediate portion so as to constitutemanifolds for distributing the fluid to the channels 36 at one end andfor collecting it from the channels at the other end.

The intermediate portion 34 is secured to a support structure 38 in theform of an annular structure of C/C composite material. It is formed bywinding superposed layers of a fiber fabric onto a mandrel and bybonding the layers together by fibers that extend transversely relativeto the layers, e.g. by needling, with the resulting annular preformbeing densified with a carbon matrix. A method of making needled annularpreforms to constitute reinforcement in structural parts made of C/Ccomposite material is described in above-mentioned patent U.S. Pat. No.4,790,052.

The support structure 38 and the intermediate portion can be made as twoparts which are assembled together by brazing, or they can be made as asingle part as in the example shown.

The heat shield 32 is brazed to the face of the intermediate portionthat presents the channels 36 and the grooves 37.

Brazing is performed as described above with reference to FIGS. 1 and 2,possibly after forming a coating of metal to which the brazing alloyadheres, and at least after forming a sealing coating on the walls ofthe channels 36 and the grooves 37.

FIG. 4 is highly diagrammatic and shows a ram jet structure having awall 40 that constitutes a heat exchanger of the invention.

The wall 40 is analogous in structure to the block 10 of FIG. 1 and itis manufactured in similar manner. The heat shield 42 situated on theinside of the wall is made of CMC material, e.g. of C/SiC. It is brazedto an intermediate portion 44 via a face having channels 46 machinedtherein, the face of the intermediate portion 44 having the channelsbeing covered by the heat shield 42. The channels 46 carry a fluidconstituting a fuel that is injected into the combustion chamber afterbeing heated by passing through the wall 40.

The intermediate portion 44 is made of C/C composite material and it isbrazed to a support structure 48 likewise made of C/C compositematerial. The support structure is advantageously in the form of ahoneycomb so as to make the assembly as light as possible.

The brazing, the optional formation of a metal coating for adhesion onthe faces that are to be brazed together, and the formation of a sealingcoating on the walls of the fluid circulation channels are all performedas described with reference to FIGS. 1 and 2.

Above, it is assumed that the fluid circulation channels are formed inthat face of the intermediate portion which is situated adjacent to theheat shield. That is a preferred disposition. Nevertheless, the channelscould be formed in that face of the intermediate portion which issituated adjacent to the support structure.

What is claimed is:
 1. A heat exchanger of composite material,characterized in that it comprises an intermediate portion of refractorycomposite material in which fluid circulation channels are formed, theintermediate portion being interposed between a portion of refractorycomposite material forming a heat shield and a portion ofthermostructural composite material forming a support structure of theheat exchanger, the portions constituting the heat exchanger beingassembled together by brazing.
 2. A heat exchanger according to claim 1,characterized in that the intermediate portion is of C/C compositematerial.
 3. A heat exchanger according to claim 1, characterized inthat the portion forming a heat shield is of ceramic matrix compositematerial.
 4. A heat exchanger according to claim 3, characterized inthat the portion forming a heat shield is of C/SiC composite material.5. A heat exchanger according to claim 1, characterized in that theportion forming a support structure is of C/C composite material.
 6. Aheat exchanger according to of claim 1, characterized in that the fluidcirculation channels are formed in one face of the intermediate portionand are defined in part by the adjacent wall of one of the other twoportions.
 7. A heat exchanger according to claim 1, characterized inthat the fluid circulation channels are provided with a leakproofcoating.
 8. A heat exchanger according to claim 1, characterized in thatthe portion forming a support structure is of honeycomb structure.
 9. Aheat exchanger according to claim 1, characterized in that the portionforming a support structure is of a composite material comprising fiberreinforcement having a plurality of superposed fiber layers bondedtogether by fibers extending transversely relative to the layers.
 10. Acombustion chamber wall element for a ram jet, characterized in that itincorporates a heat exchanger in accordance with claim
 1. 11. A methodof manufacturing a heat exchanger of composite material, the methodbeing characterized in that it comprises: making an intermediate portionof refractory composite material provided with fluid circulationchannels; making a heat shield portion of refractory composite material;making a structural portion out of thermostructural composite material;and assembling the various portions together by brazing with theintermediate portion being interposed between the heat shield portionand the structural portion.
 12. A method according to claim 11,characterized in that the fluid circulation channels are formed bymachining in a face of the intermediate portion.
 13. A method accordingto claim 11, characterized in that the intermediate portion is made ofC/C composite material.
 14. A method according to claim 11,characterized in that a leakproof coating is formed on the walls of thefluid circulation channels.
 15. A method according to claim 14,characterized in that the leakproof coating is formed by depositing alayer of metal.
 16. A method according to claim 11, characterized inthat the heat shield portion is made of ceramic matrix compositematerial.
 17. A method according to claim 11, characterized in that thestructural portion is made of C/C composite material.
 18. A methodaccording to claim 11, characterized in that a honeycomb structureportion is made.
 19. A method according to claim 11, characterized inthat the structural portion is made of a composite material having fiberreinforcement densified by a matrix, and in that the fiber reinforcementis made by superposing a plurality of fiber layers and needling themtogether.
 20. A method according to claim 11, characterized in that abrazing alloy adhesion layer of metal is formed on the facing faces ofthe portions that are to be assembled together by brazing.
 21. A methodaccording to claim 20, characterized in that the fluid circulationchannels are formed by machining in a face of the intermediate portionwhich is covered by an adjacent face of another portion, and the metallayer is formed on the face of the intermediate portion, after thechannels have been machined, and on the adjacent face, in such a mannerthat the metal layer also constitutes a coating for leakproofing thewalls of the fluid circulation channels.
 22. A heat exchanger accordingto claim 2, characterized in that: the portion forming a heat shield isof ceramic matrix composite material; the portion forming a heat shieldis of C/SiC composite material; the portion forming a support structureis of C/C composite material; the fluid circulation channels are formedin one face of the intermediate portion and are defined in part by theadjacent wall of one of the other two portions; the fluid circulationchannels are provided with a leakproof coating; the portion forming asupport structure is one of honeycomb structure; and a compositematerial comprising fiber reinforcement having a plurality of superposedfiber layers bonded together by fibers extending transversely relativeto the layers.
 23. A method according to claim 12, characterized inthat: the intermediate portion is made of C/C composite material; aleakproof coating is formed on the walls of the fluid circulationchannels depositing a layer of metal; the heat shield portion is made ofceramic matrix composite material; the structural portion is made of C/Ccomposite material; one of a honeycomb structure portion and a compositematerial having fiber reinforcement densified by a matrix, with thefiber reinforcement made by superposing a plurality of fiber layers andneedling them together is made; a brazing alloy adhesion layer of metalis formed on the facing faces of the portions that are to be assembledtogether by brazing; and the fluid circulation channels are formed bymachining in a face of the intermediate portion which is covered by anadjacent face of another portion, and the metal layer is formed on theface of the intermediate portion, after the channels have been machined,and on the adjacent face, in such a manner that the metal layer alsoconstitutes a coating for leakproofing the walls of the fluidcirculation channels.